What permits are needed for organizing a public seminar in Bangkok, especially for foreign speakers?

Question not about Visa, but I don't know where else to ask. Does anyone have experience organizing a public seminar in Bangkok? Let's say it's a seminar of a religious nature, so not looking to make a profit by selling something. And it's by an organization that's not registered in Thailand and thus doesn't have an office in Thailand. Concern is what type of permit do I need to get if the speakers are foreigners. Tried to google this topic but couldn't find anything useful. Thank you.